This invention relates to oxidized film structures, including crystalline metal oxides epitaxially grown on such structures for electronic, optical, and magnetic applications.
New epitaxial metal oxide thin film microstructures on substrates such as silicon could offer a variety of improved properties that are necessary in future microelectronics, including improved dielectric, optical, electronic, and magnetic properties. For example, as device scaling continues in the existing silicon-based complementary metal oxide semiconductor (CMOS) industry, SiO2 is approaching its fundamental limits as the gate dielectric. Alternative gate materials with higher dielectric constants than SiO2 are needed. The alkaline earth metal oxide of SrTiO3 (STO) is a promising material for this purpose because of its high bulk dielectric constant. However, direct growth of this oxide on Si is especially difficult because Si is highly reactive with oxygen, forming amorphous SiO2, or glassy silicates. Additionally, extensive interdiffusion or chemical reactions can degrade the properties of the oxide and/or the underlying silicon. It is therefore critical to produce a stable interfacial template on Si that sustains high temperatures in an oxygen environment for the subsequent single-crystal film growth.
A method of growing the alkaline earth metal oxide of BaTiO3 (BTO) on Si has been reported by McKee et al (1) (Appl. Phys. Lett. 59 (7) p. 782-784, Aug. 12, 1991). The high temperature method includes first reacting Ba metal with a clean Si surface at a temperature greater than 840xc2x0 C. to form a (2xc3x971) barium silicide submonolayer. The authors state that this barium silicide (discussed in detail by Make, R. A. and Walker, F. J., Appl. Phys. Lett. 63 (20), p. 2818-2820, Nov. 15, 1993) plays a critical role as an interfacial template between silicon and the subsequently epitaxially-grown oxide layers of BaO and BTO. Make et al (1) report growing a BaO layer on this template at temperatures ranging from room temperature to 800xc2x0 C. by the simultaneous or cyclic exposure of the barium silicide film to Ba metal and oxygen at partial pressures of 10xe2x88x927 Torr to 10xe2x88x924 Torr and that xe2x80x9cthe cyclic growth conditions are most important in optimizing the surface and film quality.xe2x80x9d BTO is then grown on this BaO layer by the shuttering of Ti and Ba cells at one monolayer intervals in an oxygen partial pressure of 2.5xc3x9710xe2x88x927 Torr.
U.S. Pat. No. 5,482,003 (Mckee and Walker) similarly discloses a high temperature method for depositing an epitaxial layer of alkaline earth oxide upon another layer having an ordered face-centered-cubic lattice structure (e.g., Si) or an alkaline earth oxide having a sodium chloride-type lattice structure (e.g., BaO, SrO, CaO, and MgO). The method includes the steps of cleaning a silicon substrate using the Modified RCA technique, forming a stable silicide layer by depositing a submonolayer of a mixture of Ba and Sr at a temperature greater than 850xc2x0 C. in an ultrahigh vacuum, oxygen-free environment (10xe2x88x9210 Torr to 10xe2x88x929 Torr), lowering the temperature to between 200xc2x0 C. and 300xc2x0 C. at which point a further mixture of Ba and Sr is deposited until the surface is covered by about one monolayer of the metal mixture, then increasing the pressure of the ultrahigh vacuum to between 1xc3x9710xe2x88x926 Torr to 5xc3x9710xe2x88x926 Torr with the introduction of oxygen and then exposing the metal-covered surface to this oxygen and additional amount of the metal mixture to epitaxially grow alkaline earth metal oxide on the silicon surface. Further layers of oxide are subsequently grown on this layer. A similar method for growing STO on Si (100) has also been reported by Mckeee et al (2) (Phys. Rev. Lett. 81 (14), p. 3014-3017, Oct. 5, 1998).
A method of growing STO on Si has been reported by Eisenbeiser et al (Appl. Phys. Lett. 76 (10) p. 1324-1326, Mar. 6, 2000) and Yu et al (1) (Mater. Res. Soc. Symp. Proc. 567 p.427-433, 1999). Eisenbeiser et al discloses a method using lower temperatures and having fewer steps than the Make et al methods. In Eisenbeiser et al, metallic Sr was reacted with the silicon oxide on the surface of a silicon substrate at a temperature greater than 700xc2x0 C. under high vacuum to produce a 2xc3x971 surface reconstruction. STO was subsequently directly grown on this layer in the temperature range of 200xc2x0 C. to 800xc2x0 C. and up to 10xe2x88x925 Torr oxygen partial pressure.
Yu et al (2) (J. Vac. Sci. Technol. B 18 (4) p. 2139-2145, July/August 2000) further reports a similar low temperature method of growing STO or BTO on Si. Yu et al (2) discloses a method whereby metallic Ba or Sr was reacted with as-received commercial Si(001) wafers at a temperature below 800xc2x0 C. at a pressure below mid-10xe2x88x9210 Torr. BTO and STO films were subsequently deposited at a temperature in the range of 200xc2x0 C. to 700xc2x0 C. under up to 10xe2x88x925 Torr oxygen partial pressure. U.S. Pat. No. 6,113,690 (Yu et al) discloses another similar method whereby an alkaline earth metal, preferably Ba or Sr, was reacted with a silicon substrate having a silicon dioxide layer at a temperature in the range of 700xc2x0 C. to 800xc2x0 C. and a pressure in the range of 10xe2x88x9210 Torr to 10xe2x88x929 Torr.
U.S. Pat. No. 6,022,410 (Yu et a) discloses a low temperature method of forming an ordered Si wafer surface for subsequent thin film epitaxy. The low temperature method includes reacting atomic beams of one or more alkaline earth metals and an atomic beam of Si with a clean Si surface at a temperature in the range of 500xc2x0 C. to 750xc2x0 C. to form a single crystal alkaline earth metal silicide layer (e.g., BaSi2) on the surface of the silicon substrate.
Accordingly, there is a need for a simplified process that produces a high quality, crystalline oxide structure for semiconductor applications as well as for other electronic, optical, and magnetic applications.
The present invention encompasses a stable oxidized film structure and an improved method of making such a structure, including an improved method of making an interfacial template for growing a crystalline metal oxide structure. The improved method comprises the steps of providing a substrate with a clean surface and depositing a metal on the surface at a high temperature while under a vacuum to form a metal-substrate compound layer on the surface with a thickness of less than one monolayer. The compound layer is then oxidized by exposing the compound layer to essentially oxygen at a low partial pressure and low temperature. The method may further comprise the step of annealing the surface while under a vacuum to further stabilize the oxidized film structure. A crystalline metal oxide structure may be subsequently epitaxially grown by using the oxidized film structure as an interfacial template and depositing on the interfacial template at least one layer of a crystalline metal oxide.
It is an object of the present invention to provide an improved method for making a high quality, robust, and stable oxidized film structure.
It is a further object of the present invention to provide an interfacial template and an improved method for preparing the interfacial template that can sustain high temperatures in an oxygen environment for subsequent epitaxial growth of a crystalline metal oxide structure.
It is a further object of the present invention to provide an improved method for producing an alkaline earth metal oxide structure that can function as a gate material with a higher dielectric constant than SiO2 for semiconductor applications.
It is a further object of the present invention to provide an improved method for producing an alkaline earth metal oxide structure incorporating a silicon substrate.
It is a further object of the present invention to provide an epitaxy method that can be routinely performed in conventional processing systems or growth chambers.
It is a further object of the present invention to provide an epitaxy method that can be conveniently scaled-up.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.